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  <div class="section" id="fix-buoyancy-command">
<span id="index-0"></span><h1>fix buoyancy command<a class="headerlink" href="#fix-buoyancy-command" title="Permalink to this headline">¶</a></h1>
<div class="section" id="syntax">
<h2>Syntax<a class="headerlink" href="#syntax" title="Permalink to this headline">¶</a></h2>
<div class="highlight-python"><div class="highlight"><pre>fix ID group-ID buoyancy keyword value ...
</pre></div>
</div>
<ul class="simple">
<li>ID, group-ID are documented in <a class="reference internal" href="fix.html"><em>fix</em></a> command</li>
<li>buoyancy = style name of this fix command</li>
<li>zero or more keyword/value pairs may be appended to args; the <em>dim</em> keyword must be used</li>
<li>keyword = <em>density</em> or <em>dim</em> or <em>level</em> or <em>region</em></li>
</ul>
<pre class="literal-block">
<em>density</em> value = density-value
  density-value = fluid density in mass/length^3 units
<em>dim</em> value = x or y or z
  x, y, z = define the water level as x=const, y=const, z=const plane
<em>level</em> value = lev
  lev = water level in length units, along the axis defined by dim
<em>region</em> value = region-ID
  region-ID = ID of region atoms must be in to have added buoyancy force
</pre>
</div>
<div class="section" id="examples">
<h2>Examples<a class="headerlink" href="#examples" title="Permalink to this headline">¶</a></h2>
<div class="highlight-python"><div class="highlight"><pre>fix bu all buoyancy level 0.06 dim z density 1000
</pre></div>
</div>
</div>
<div class="section" id="description">
<h2>Description<a class="headerlink" href="#description" title="Permalink to this headline">¶</a></h2>
<p>Add a buoyancy force for each atom in the group. The water level is
assumed to be a x=const, y=const or z=const plane, where the axis
is defined via the <em>dim</em> keyword and const is defined via the <em>level</em>
keyword. E.g. <em>dim</em> = x and <em>level</em> = 0.1 would define the water level
as x = 0.1.</p>
<p>The buoyancy force is equivalent to the weight of the displaced fluid.
Thus, if the particle is fully submerged, the buoyancy force is equal
to particle volume * fluid density. If the particle is not submerged,
the buoyancy force is 0. If the particle is partially submerged, the
fix will calculate the force based on the submerged particle volume.
The fluid density is defined by keyword <em>density</em>.</p>
<p>As buoyancy is triggered by static pressure difference in a fluid usually
caused by gravity, this fix requires to use a <a class="reference internal" href="fix_gravity.html"><em>fix gravity</em></a>.
The gravity vector has to be specified in the same axis as in this fix
(via the <em>dim</em> keyword).</p>
<p>If the <em>region</em> keyword is used, the atom must also be in the
specified geometric <a class="reference internal" href="region.html"><em>region</em></a> in order to have buoyancy force
added to it.</p>
</div>
<hr class="docutils" />
<div class="section" id="restart-fix-modify-output-run-start-stop-minimize-info">
<h2>Restart, fix_modify, output, run start/stop, minimize info<a class="headerlink" href="#restart-fix-modify-output-run-start-stop-minimize-info" title="Permalink to this headline">¶</a></h2>
<p>No information about this fix is written to <a class="reference internal" href="restart.html"><em>binary restart files</em></a>. No <a class="reference internal" href="fix_modify.html"><em>fix_modify</em></a> option applies
to this fix. This fix computes a a global 3-vector of the total
buoyancy force, which can be accessed by various <a class="reference internal" href="Section_howto.html#howto-8"><span>output commands</span></a>.
No parameter of this fix can be used with the <em>start/stop</em> keywords of
the <a class="reference internal" href="run.html"><em>run</em></a> command.</p>
<p>The forces due to this fix are imposed during an energy minimization,
invoked by the <code class="xref doc docutils literal"><span class="pre">minimize</span></code> command.</p>
</div>
<div class="section" id="restrictions">
<h2>Restrictions<a class="headerlink" href="#restrictions" title="Permalink to this headline">¶</a></h2>
<p>This fix requires to use a <a class="reference internal" href="fix_gravity.html"><em>fix gravity</em></a>. The gravity
vector has to be specified in the same axis as in this fix (via the
<em>dim</em> keyword).</p>
</div>
<div class="section" id="related-commands">
<h2>Related commands<a class="headerlink" href="#related-commands" title="Permalink to this headline">¶</a></h2>
<p><a class="reference internal" href="fix_setforce.html"><em>fix setforce</em></a>, <a class="reference internal" href="fix_aveforce.html"><em>fix aveforce</em></a>,
<a class="reference internal" href="fix_addforce.html"><em>fix addforce</em></a></p>
</div>
<div class="section" id="default">
<h2>Default<a class="headerlink" href="#default" title="Permalink to this headline">¶</a></h2>
<p><em>level</em> = 0, <em>density</em> = 0</p>
</div>
</div>


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